The present invention relates to a phase shifter circuit, and, more particularly, to a phase shifter circuit used in a mixer or a modulator of radio communication apparatus, such as a cellular telephone.
FIG. 1 is a schematic circuit diagram of a conventional phase shifter circuit 10.
The phase shifter circuit 10 comprises a first differential amplifier 11 that receives an input signal (analog frequency signal) Sin and generates first and second phase shift signals S1, S2 having a phase difference of 180 degrees (e.g. 0xc2x0 and 180xc2x0) with each other and a second differential amplifier 12 that receives an input signal Sin and generates third and fourth phase shift signals S3, S4 having the phase difference of 180 degrees (e.g. 90xc2x0 and 270xc2x0) with each other. FIG. 3 is a graph showing the relationship between frequency and phase in each of the phase shift signals. As shown in FIG. 3, each of the phase differences between the phase shift signals S1, S3, between the phase shift signals S2, S3, between the phase shift signals S2, S4 and between the phase shift signals S4, S1 is 90 degrees. The graph shows that the phase shifter circuit 10 holds the phase differences of the respective phase shift signals S1 to S4 at 90 degrees at any frequency.
FIG. 2 is a graph showing the relationship between frequency and amplitude in each of the phase shift signals. As shown in this graph, however, the conventional phase shifter circuit 10 matches the amplitude of the first and second phase shift signals S1, S2 and the amplitude of the third and fourth phase shift signals S3, S4 only at a certain frequency (f0).
The phase shifter circuit 10, for example, as shown in FIG. 4, is applied to a mixer circuit 20 for a radio communication apparatus which switches a plurality of IF frequencies. A phase shifter circuit 10a receives an intermediate frequency signal IFin as the input signal Sin and generates first to fourth intermediate frequency signals. A limit amplifier 21a for matching the amplitude of the first to fourth intermediate frequency signals at a plurality of frequencies is connected to the phase shifter circuit 10a. A phase shifter circuit 10b receives a local oscillation signal LOin as the input signal Sin and generates first to fourth local oscillation signals. A limit amplifier 21b for matching the amplitude of the first to fourth local oscillation signals at a plurality of frequencies is connected to the phase shifter circuit 10b. Accordingly, the first to fourth intermediate frequency signals and first to fourth local oscillation signals whose amplitudes are balanced are supplied to modulation mixers 22a, 22b. However, use of the limit amplifiers 21a, 21b increases the circuit area and power consumption of the mixer circuit 20.
An object of the present invention is to provide a phase shifter circuit which generates a phase shift signal whose amplitude matches at a plurality of frequencies without increasing the circuit area.
In a first aspect of the present invention, a phase shifter circuit is provided that includes a first differential amplifier for receiving a first input signal having a first frequency and generating a first phase shift signal having a first amplitude and a second differential amplifier for receiving the first input signal and generating a second phase shift signal having a phase difference of 90 degrees between the first and second phase signals and substantially the same amplitude as the first amplitude. At least one third differential amplifier is connected in parallel to the first differential amplifier to receive a second input signal having a second frequency that is different from the first frequency and generate a third phase shift signal having substantially the same amplitude as the first amplitude. The first differential amplifier is activated in accordance with the first frequency and the at least one third differential amplifier is activated in accordance with the second frequency.
In a second aspect of the present invention, a phase shifter circuit is provided that includes a first differential amplifier for receiving a first input signal having a first frequency and generates a first phase shift signal having a first amplitude and a second differential amplifier for receiving the first input signal and generates a second phase shift signal having a phase difference of 90 degrees between the first and second phase shift signals and substantially the same amplitude as the first amplitude. At least one third differential amplifier is connected in parallel to the first differential amplifier to receive a second input signal having a second frequency that is different from the first frequency and generate a third phase shift signal having substantially the same amplitude as the first amplitude. At least one fourth differential amplifier is connected in parallel to the second differential amplifier to receive the second input signal and generate a fourth phase shift signal having a phase difference of 90 degrees between the second and third phase shift signal and substantially the same amplitude as the first amplitude. The first and second differential amplifier are activated in accordance with the first frequency and the at least one third differential amplifier and the at least one fourth differential amplifier are activated in accordance with the second frequency.
In a third aspect of the present invention, a phase shifter circuit is provided that includes a first differential amplifier for receiving an input signal having a predetermined frequency and generating a first phase shift signal in accordance with a first predetermined gain and a second differential amplifier for receiving the input signal and generating a second phase shift signal having a phase difference of 90 degrees between the first and second phase shift signals in accordance with a second predetermined gain. A control circuit is connected to the first and second differential amplifiers to receive the input signal and control the first and second predetermined gains of the first and second differential amplifiers based on an amplitude of the input signal at the predetermined frequency of the input signal.
In a fourth aspect of the present invention, a phase shifter circuit is provided that includes a first differential amplifier for receiving an input signal having a predetermined frequency and generating a first phase shift signal in accordance with a first gain in response to a first control signal and a second differential amplifier for receiving the input signal and generating a second phase shift signal having a phase difference of 90 degrees between the first and second phase shift signals in accordance with a second gain in response to the first control signal. A third differential amplifier is connected to the first differential amplifier to receive the input signal and generate a third phase shift signal in accordance with a third gain that is different from the first gain in response to a second control signal. A fourth differential amplifier is connected to the second differential amplifier to receive the input signal and generate a fourth phase shift signal in accordance with a fourth gain that is different from the second gain in response to the second control signal. A control circuit is connected to the first to fourth differential amplifiers to receive the input signal and selectively supply the first and second control signals to the first to fourth differential amplifiers based on an amplitude of the input signal at the predetermined frequency of the input signal.
In a fifth aspect of the present invention, a control circuit of a phase shifter circuit for controlling a predetermined gain of the phase shifter circuit is provided. The phase shifter circuit receives an input signal having a predetermined frequency and generates first and second phase shift signals having a phase difference of 90 degrees in accordance with the predetermined gain. The control circuit includes a control signal generation circuit for receiving the input signal and generating a control signal for controlling the predetermined gain of the phase shifter circuit based on an amplitude of the input signal at a predetermined frequency of the input signal.
In a sixth aspect of the present invention, a method for controlling a predetermined gain of a phase shifter circuit is provided. The phase shifter circuit receives an input signal having a predetermined frequency and generates first and second phase shift signals having a phase difference of 90 degrees from each other in accordance with the predetermined gain. First, a frequency-amplitude signal having an amplitude corresponding to the predetermined frequency of the input signal is generated. Then, the predetermined gain of the phase shifter circuit is controlled based on the amplitude of the frequency-amplitude signal.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.